A number of known control arrangements regulate the position or elevation of implements, such as plows, attached to or drawn by agricultural vehicles, such as tractors. Such control systems generally sense the position of a three-point hitch or other implement positioning structure and compare this position to a command value set by an operator using a command device. Based upon this comparison, such control systems generate a control signal for an actuator to vertically move the hitch, along with the implement mounted on it, to the desired elevation.
Known control systems can also operate based upon the draft or load force generated by the interaction of an implement with the ground. Such draft force can be generated by the implement penetrating the ground, or by the implement being engaged with the ground. The control system typically compares the sensed draft force to a command value set by an operator and generates a control signal for an actuator to vertically move the hitch to maintain the desired draft force.
When the position of a hitch is controlled based upon a comparison between the actual hitch position and a commanded position, the motion of the hitch which occurs as a result of the operator changing the commanded position depends on several factors. The factors include the weight of the implement, the desired drop or raise rate of the actuator which may be adjusted by the operator using a drop or raise rate knob or device, and the actual movement of the device used by the operator to set the command value.
Known control systems, however, may experience a problem which causes discontinuous or jerky movement of the implement, and decreases the precision with which the motion and position of the implement are controlled. The problem may be particularly troublesome when the implement being controlled is relatively heavy and the movement commanded by the operator is relatively small. The problem occurs because, once the actuator or valve feeding the actuator with pressurized fluid begins to respond to a change in the position command device, the implement drops or rises quickly before the control system gains control over the implement. Once control is attained, the implement may have already dropped or risen beyond the small distance commanded by the operator, thereby resulting in a jerk and imprecise positioning.